The use of prepreg (i.e., fibrous material impregnated with uncured thermosettable resin) to form composite articles of manufacture is widespread. While prepreg is used in the airline industry to form various structural airplane parts, it also has significant application in the production of ductwork used on board an airplane.
The current manufacturing process for forming composite prepreg ducts of substantially circular cross section is to lay up plies of prepreg on a disposable plaster mandrel, the mandrel having been pre-coated with a parting substance to prevent the prepreg from sticking. A release film is then placed over the outermost ply of prepreg. Heat-shrinkable tape is then manually spirally wound over the release film, the tape having a certain amount of overlap when it is wound. Manual wrapping of the heat-shrinkable tape is an extremely labor-intensive process, and is thereby a significant manufacturing cost. The mandrel, laid up with prepreg and release film covered with heat-shrinkable tape, is then placed within an airtight bag to which a vacuum is applied, the vacuum serving to compact the prepreg plies.
The assembly of mandrel and radially outward-lying components is normally then placed in an oven and the application of heat causes the tape to shrink. Prior to introduction into the oven, a series of perforations are placed in the heat-shrinkable tape to allow excess resin to bleed through the tape during the heating process. The oven heat cures the resin and the simultaneous contraction of the tape consolidates the prepreg and eliminates or minimizes voids therebetween.
After the curing process is complete and adequate cooling has occurred, the contracted tape and release film are cut away and discarded. A hammer is applied to the outermost surface of the cured prepreg composite to break the internal disposable plaster mandrel, thereby leaving the desired end product, a composite prepreg duct. Because the cured prepreg composite may be slightly porous, a sealant is normally wiped or sprayed on the interior surface of the duct to provide an airtight seal between the interior and exterior of the duct.
When structural airplane parts (or ducts) of non-circular cross section are produced, the process is identical to that for producing ducts of circular cross section, except that heat-shrinkable tape may not be used. Instead, tape composed of an essentially non-shrinkable material (e.g., polyvinyl alcohol) is used to prevent the deformation and surface wrinkling of the part that would occur with heat-shrinkable tape.
As a result, there has been a long-felt need for a method of producing composite prepreg articles that does not possess the labor-intensive, tape-wrapping step of current manufacturing processes. The need is particularly strong in the airline industry, where extensive manufacturing costs associated with taping are incurred in the production of composite prepreg ducts and other structural parts. This invention is directed to satisfying this need.